1. Field of the Invention
The present invention relates to reverse osmosis water purification systems and pumping systems for pumping water to an osmosis membrane.
2. Description of Related Art
Water purification is becoming increasingly popular among todays health conscious consumers. One type of purification process includes pumping tap water through an osmotic membrane that filters out impurities in the water. The membranes typically produce a large pressure drop and therefore require a high inlet pressure to push the water through the membrane. Some municipal water systems do not provide a line pressure that is high enough to be used with an osmotic membrane and therefore a pump must be interposed between the reverse osmosis (RO) unit and water source to increase the pressure of the water to a predetermined operating level. Typically the suction side of the pump is connected directly to a water faucet, wherein the pump and RO unit are a detachable unit. To obtain the high pressures necessary it is preferable to use a positive displacement pump. Available pumps today have been designed with economy and efficiency in mind and thus utilize a DC motor operating in its most efficient speed, typically 1500 to 6000 RPM. Since it takes the same power to do a given amount of work whether the pumps operate at high or low voltages, the current needed to drive a low voltage motor is higher than the corresponding current for a high voltage motor. To minimize unnecessary power losses, low voltage motors are typically fitted with low resistance copper brushes. This type of pump produces a flow rate greater than the flow through the membrane and therefore an accumulator is typically attached in line, to store the excess water produced by the pump under pressure to reduce the on-off cycling of the pump.
The pump turns on when the inlet pressure at the RO unit drops below a predetermined level. The pump supplies water to the RO unit and accumulator until a predetermined upper pressure limit is reached at which point the pump turns off. The accumulator then supplies the osmotic membrane with water until the pressure at the RO unit inlet drops below the lower limit. Typical pumps have a flow rate ten times the flow rate through the membrane, wherein the pump is actually operating less than ten percent of the time. The constant starting of the pump is noisy and tends to reduce the life of the pump. Additionally, pressure switches must be incorporated to measure the water pressure on the output of the pump and to start and stop the pump when the respective lower and upper pressure limits are reached. The pressure switches have slow response times and are subject to fatigue. A pump that is used in present reverse osmosis water treatment systems is disclosed in U.S. Pat. No. 4,610,605 issued to Hartley. The Hartley device is a positive displacement pump with a pressure switch that starts and stops the pump motor based on the fluid pressure on the output port of the pump. Although this pump is acceptable, what is desired is a reverse osmosis system and pump that does not constantly cycle the pump or require a bulky accumulator.
Operating a pump at say typically 20% duty cycle for a continuous duty requirement such as a filtration membrane results in a power requirement during the operational mode that is five times that of a continuously operating pump. The power supply thus has to be sized for the actual operating requirement resulting in the usage of a large component with high no-load losses.
The pump motors typically used with present pumps are permanent magnet DC motors operating on 24 Volt full wave rectified and filtered 60 hertz power. When a DC motor is started, its initial impedance is equal to its winding resistance, resulting in a high starting current that slowly abates as the motor picks up speed and inductance from the changing field is created. The motors currently used run into problems when started and stopped frequently in that their windings tend to overheat, shortening the overall life of the motor. Also the copper brushes running against the copper commutators create equal wear on both surfaces forcing a costly replacement of the armature when replacing worn brushes. These motors run into problems when used with pumps having pressure switches, in that if the voltage supplied to the motor drops while the pressure switch is closed (when the pump is running) the electric motor will stall, which can damage and sometimes destroy the motor. It is thus desirable to have a pump motor that will not stall under varying load conditions.